1. Field of the Invention
The present invention relates to capillary tubes for holding an optical fiber, and more specifically, to a capillary tube for holding an optical fiber, in which an insertion hole for inserting and fixing an optical fiber is formed. An outer circumferential surface is constituted as a cylindrical surface, and a region that serves as a mark is formed at a predetermined position in a circumferential direction of the cylindrical surface.
2. Description of the Related Art
As well known in the related art, the capillary tube for holding an optical fiber (hereinafter also simply referred to as capillary tube) normally includes an insertion hole to which one or a plurality of optical fibers is inserted and fixed at a periphery of a central axis, and is formed as a columnar body in which an outer circumferential surface is the cylindrical surface. This type of capillary tube is faced to and connected to an optical component in a circular inner hole (hole in which an inner circumferential surface is constituted as a cylindrical surface) of an external cylinder tube represented by a cylindrical tube, where the opposing end surface of the capillary tube is angle polished with the opposing end surface of the optical component to reduce the insertion loss of light in such case. This capillary tube is normally caused to adhere and fixed in the circular inner hole of the external cylinder tube by an adhesive.
In a case of angle polishing the end surface of the capillary tube, the tilt direction needs to be accurate, and thus the region that serves as a mark for visually recognizing the tilt direction is formed at a predetermined position in the circumferential direction of the capillary tube. Specifically, in the related art, one portion of the cylindrical surface of the capillary tube is formed as a flat portion, and the tilt direction in angle polishing is visually recognized with such flat portion as the mark. In this case, it is not easy to visually recognize the flat portion in distinction from the cylindrical surface, and thus the large dimension in the width direction of the flat portion needs to be set for ensuring accuracy in visual recognition.
However, the lacking portion of the volume of the capillary tube is large if the large dimension in the width direction of the flat portion is set, and thus the amount of adhesive filled between the inner circumferential surface of the external cylinder tube and the outer circumferential surface of the capillary tube becomes partially in excess. When the cured adhesive expands or contracts due to temperature change and the like, there are problems that the excessive adhesive may push the capillary tube thereby degrading the optical characteristics or the adhesive may delaminate due to thermal expansion difference.
JP 10-73742 A does not aim to avoid such problems described above caused by the adhesive but discloses formation of a groove of V-shaped cross-section instead of a flat portion at the cylindrical surface of the capillary tube. According to the method of forming the groove at the cylindrical surface of the capillary tube, the existing drawbacks that arise when the filled amount of adhesive is partially excessive can be expected to be avoided when such capillary tube is inserted to the external cylinder tube such as the cylindrical tube, and then caused to adhere and fixed thereto.
However, the capillary tube disclosed JP 10-73742 A described above merely aims to form the groove of V-shaped cross-section at the cylindrical surface as a mark, and thus a projecting angulated portion exists between the cylindrical surface and both inner side surfaces of the groove and a recessed corner portion exists at the bottom of the groove.
If the projecting angulated portion or the recessed corner portion (in particular, projecting angulated portion) exists at the outer circumferential surface of the capillary tube, chips, cracks, and the like tend to easily occur when handling, conveying, or transporting the capillary tube. Further, even when the circular inner hole of the external cylinder tube is filled with adhesive and the external cylinder tube and the capillary tube are relatively rotated for positioning in the circumferential direction, there is a risk that chips, cracks, and the like may occur at the angulated portion due to resistance and the like at the time of slidable movement or the rotation of the external cylinder tube and the capillary tube. Moreover, glass fragments may mix into the adhesive due to occurrence of chips, cracks, and the like. The glass fragments inhibit the relative rotation of the external cylinder tube and the capillary tube, which more easily causes chips and the like to occur.
Further, when the adhesive is filled between the external cylinder tube and the capillary tube, and the external cylinder tube and the capillary tube are relatively rotated as described above, a crucial problem that air bubbles may mix into the adhesive may arise when the resistance at the time of rotation becomes higher or inappropriate stirring is performed due to the existence of a plurality of angulated portions and corner portions at the outer circumferential surface of the capillary tube.